Wind turbines create power proportional to the swept area of their blades. Increasing the length of a wind turbine's blades increases the swept area, which produces more power. A wind turbine's generator, gears, bearings, and support structure must be designed around the expected wind load and power production. At low wind speeds very long blades are desirable to get as much power as possible out of the available wind. At high wind speeds a wind turbine must control the power production and the mechanical loads developed. Eventually, if the wind speed becomes high enough, the turbine must shut down to avoid damaging components, so short blades are desirable to keep the turbine producing power in high winds.
The choice of a rotor diameter for a wind turbine is a design trade-off between energy production in low winds and load limitation in high winds. Wind turbine manufacturers often sell a variety of rotor sizes for a given wind turbine model. The rotor sizes are optimized for sites that have a low, medium, or high annual average wind speed. However, the rotor size selected is always a compromise, and there are conditions in which the turbine does not perform optimally because the rotor is too big or too small.
Disclosed in U.S. Pat. No. 6,902,370 issued Jun. 7, 2005 to Dawson, et al., entitled “Telescoping Wind Turbine Blade,” and incorporated by reference herein in its entirety, is a variable length wind turbine blade that allows for a large diameter in low winds and a small diameter in high winds. This is accomplished by having a root portion and a tip portion of the blades. The tip portion may be extended or retracted relative to the root portion, depending on the amount of wind present.
A sharp transition between the tip portion and the root portion reduces aerodynamic efficiency and generates aerodynamic noise. Thus, it would be desirable to provide a smooth transition between the two blade portions.
Wind turbines, especially when using retractable blade technology, require significant amounts of monitoring and controlling. For example, the extension and retraction of the tip portions of the blades must be monitored and closely controlled to ensure that all the blades extend and retract at the same time so as to prevent an unbalanced rotor. Thus, it would be advantageous to have a method of measuring the position of the tip portion of a blade. It would also be useful to sense other parameters that provide information about the operating conditions of the blades and wind turbine.
Because of the dust and debris in the wind, turbine blades often need cleaning. In the winter, blades have a tendency to get ice build-up. Especially in the case of a variable length blade, de-icing is important because the tip portion of the blade must retract into the root portion. Thus, it would be advantageous to have a method of cleaning and de-icing blades on a variable length wind turbine.